1. Field of Invention
Gelling agents are used in foods for a variety of purposes. As well as in familiar gelled foods, such as table jellies and similar desserts, confectionery jellies and jams, gelling agents are also valuable in meat products, and in the stabilization of emulsions in, for example, salad dressings.
2. Description of Related Art
There are two major types of food gel: those which set reversibly on cooling, and those which set irreversibly on heating. The former rely mainly on polysaccharide gelling agents, such as carrageenan, while the latter typically involve proteins, such as egg-white. Gelatin is an unusual gelling agent in that, although it consists of a mixture of proteins, its behavior is more typical of that of polysaccharides. Thus, gelatin solutions set to gels on cooling and these can be re-melted on heating.
This invention is concerned with gels which set reversibly on cooling.
A range of gelling agents is available to the food-manufacturing industry, and the selection of a suitable gelling agent for a particular application depends on many factors. Gelling agents differ in price, availability and usage rate, and also in the conditions under which they can be used and, therefore, the types of food in which they may be suitable. For example, native citrus pectin (with a high methyl ester content) forms a clear gel in the presence of acid and high concentrations of sugar, and is therefore exploited in jams and fruit jellies. Low methoxy pectin can be made to gel in the presence of calcium ions and does not require the presence of sugar; it is therefore suitable for use in reduced calorie products.
Despite this variety of gelling agents, gelatin remains unique because of its melting behavior. Gelatin gels melt at around 30.degree.-35.degree. C., which is below body temperature (37.degree. C.). Thus gelatin gels melt in the mouth, and this property has implications for improved flavor release as well as texture. The approximate melting properties of some polysaccharide-type gels are shown below.
______________________________________ Melting Properties of Polysaccharide-Type Gels Melting Temperature Gelling Agent .degree.C. ______________________________________ Cornstarch 96 Kappa-carrageenan 65 Alginate 100 Low Methoxy Pectin 65 Gelatin 30-35 ______________________________________
Another unusual feature of gelatin is that it is derived from animal sources (usually bones or hides), rather than from plants or seaweed or by bacterial fermentation. For food use, this is unacceptable to vegetarians. Furthermore, certain ethnic and religious groups cannot consume pork or pig products and may avoid any gelatin-containing food where the source of the gelatin is not specified as non-porcine. Gelatin is also an expensive product and forms gels only at relatively high concentrations. There is, thus, a need for a gelling agent which has the gelling properties of gelatin, which is capable of producing gels having melting properties similar to those of gelatin gels but which is derived from a non-mammalian source and which is capable of forming gels at low usage levels.
Xanthan is a high molecular weight polysaccharide by fermentation of the bacterium Xanthomonas campestris. Its high viscosity in aqueous solution is exploited by the food industry, particularly in the stabilization of emulsions, and in the oil industry in the production of drilling muds. Although xanthan solutions have a yield stress at zero shear rate, they will flow as viscous liquids if this stress is exceeded; thus, they do not form true gels. Xanthan has a cellulosic backbone, comprised of .beta.1,4-linked glucose residues. To alternate glucose units is attached a trisaccharide sidechain containing a mannose, a glucuronic acid, and a second mannose residue. The first mannose is acetylated, while approximately half of the terminal mannose residues of the sidechains contain a pyruvate substituent. The pyruvate content of food-grade xanthan forms a part of its legal definition in the USA and the EC.
Locust Bean Gum (LBG; also known as Carob, or Carob Gum) is a polysaccharide derived from the endosperm of the Locust Bean (Ceratonia siliqua), which grows in Mediterranean regions. It is employed by the food industry as a thickener/stabilizer, for example in ice cream, but it too does not form gels on its own. LBG is a galactomannan, having a backbone consisting of .beta.1,4-linked mannose residues with single .alpha.-1,6-linked galactose side units.
LBG does, however, interact with some other polysaccharides to give valuable synergistic effects. Thus, the seaweed polysaccharides, kappa-carrageenan and agar, which are exploited as gelling agents by the food industry, form stronger gels if mixed with LBG. Replacement of 50% of the carrageenan with LBG gives a stronger gel with a more elastic texture, and, depending on the relative prices of kappa-carrageenan and LBG at any given time, may give significant cost savings.
LBG also interacts with xanthan. In this case, although neither of the polysaccharides gels on its own, the mixture of the two forms a strong, rubbery gel. Again, the fact of gelation when mixed with LBG forms a part of the legal definition of food-grade xanthan in the USA. The interaction, and therefore the gelation of the mixture, is believed to involve unsubstituted (i.e., galactose-free) regions of the LBG galactomannan aligning themselves with the cellulose backbone of the xanthan to create a three dimensional polysaccharide network (Dea et al, 1972; McCleary, 1979). Xanthan/LBG gels, however, have a melting temperature far in excess of the melting temperature of gelatin gels, and typically in the range of from 50.degree.-60.degree. C. depending on the method by which the xanthan-producing organism, Xanthomonas campestris, has been treated to yield the xanthan. Gels produced using the xanthan/LBG system have an unpleasant rubbery and elastic texture.
Other mixtures of polysaccharides are known to form gels in aqueous solution. Examples of such mixtures include blends of alginate with pectin and of konjac mannan with agar, kappa-carrageenan or xanthan. As in the case of xanthan/LBG gels, however, the melting temperature of the resulting gels is significantly higher than that of gelatin gels and they cannot be used to form so-called melt-in-the-mouth gels.
In the production of polysaccharides (or gelatin) for use as gelling agents (or thickeners) great efforts are generally made to preserve, as far as possible, the high molecular weight of the polymers involved. In general, the reduction of the molecular size of such a polymer causes a deterioration in the gel strength obtained (or, in the case of a thickener, in the viscosity achieved) and, as a consequence, a loss in commercial value. We have, however, discovered that by reducing the molecular size in certain polysaccharide systems not only is there a fall-off in gel strength obtained but also, surprisingly, a lowering of the melting temperature of gels produced is achieved. Furthermore, the decrease in molecular size also causes a favorable decrease in the elasticity of the gels.
Hence, by the present invention, commercially useful polysaccharide gelling agents are obtained which are capable of producing gels having desirable melt-in-the-mouth properties, including gel strengths and elasticity similar to those of gelatin gels.